1. Field of the Invention
The present invention relates to a honeycomb structure in which a plurality of honeycomb segments are integrally bonded to one another by bonding material layers. More particularly, it relates to a honeycomb structure in which generation of defects such as cracks due to a thermal stress is effectively prevented during use and regeneration of a collection filter for an exhaust gas, especially a diesel particulate filter (DPF) to collect particulates and the like in the exhaust gas of a diesel engine.
2. Description of the Related Art
A honeycomb structure is incorporated and used as a collection filter for an exhaust gas, for example, a diesel particulate filter (DPF) in an exhaust system or the like of a diesel engine in order to collect and remove particulates included in an exhaust gas from a diesel engine or the like. Such a honeycomb structure has a disadvantage that a temperature rise in the whole honeycomb structure easily becomes non-uniform, and defects such as cracks are generated owing to a thermal stress during use (when the particulates are collected and removed) and during regeneration (when the particulates deposited in the filter are burnt and removed in order to eliminate an increase of a pressure loss due to the particulates deposited in the filter with an elapse of time). For the sake of the elimination of such a disadvantage, there has been proposed a honeycomb structure constituted as a bonded honeycomb segment article in which a plurality of honeycomb segments are integrally bonded on mutual bonding surfaces thereof via bonding material layers to thereby reduce the thermal stress.
However, a honeycomb structure of silicon carbide (SiC) has a merit of an excellent thermal resistance, and its use is expected. However, the honeycomb structure of silicon carbide has shortcomings that its thermal expansion coefficient is larger than that of a honeycomb structure made of a cordierite and it is poor in a thermal shock resistance. Therefore, there is a disadvantage that it is difficult to sufficiently utilize the merit. In recent years, with enlargement of the filter, the thermal stress generated during the use and the regeneration of the filter has increased more remarkably than before, with the result that a frequency of the generation of the defect based on the thermal stress and a degree of the defect have rapidly become serious.
To cope with such a disadvantage, there is disclosed a method of manufacturing a ceramic structure (honeycomb structure) including steps of forming an adhesive paste layer (bonding material layer) on a side surface of a porous ceramic member (honeycomb segment) so that the layer occupies 60% or more of a total area of this side surface; then repeating a step of laminating another porous ceramic member; and assembling a ceramic block (see Patent Document 1).
However, in the ceramic structure (honeycomb structure) disclosed in Patent Document 1, a rigid bonding structure conversely increases the thermal stress generated from a temperature gradient. Moreover, a thermal capacity of a bonding member is large and a temperature rise/drop speed is low as compared with the porous ceramic member. Therefore, for a reason that the temperature gradient in the filter is easily increased, the generation of the defect due to the thermal stress cannot necessarily sufficiently be prevented.
Moreover, when the honeycomb structure is used in the DPF, there is a problem that owing to the temperature gradient during the burning of the deposited soot, especially in forced regeneration of the filter due to excessively deposited soot, a local thermal stress is generated in a gas inflow axis direction of the honeycomb structure, a pull crack (a ring of crack) is generated in a ring direction of a honeycomb substrate, and the DPF is destroyed. Since this ring of crack is generated, a limit of the crack generated at a temperature lower than a heat-resistant limit temperature of a material of the honeycomb substrate is a soot deposit limit, and a performance of the material of the honeycomb substrate cannot sufficiently be utilized.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-219317